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Finding Youth fares to Tokyo can be very difficult as they are quite rare. Currently the Tokyo route is not operating daily anymore and SAS is closing some of the underlying fare buckets which are necessary for Youth fares to be available. This does not mean it is impossible, just recently a friends of mine booked a round trip youth fare from CPH to HND in April for travel last August. He may have been incredibly lucky with his timing though as in that instance the youth fares were only available for a few days and very gone very quickly.